Medical treatment of several illnesses requires continuous drug infusion into various body compartments via subcutaneous or intra-venous injections. Diabetes mellitus patients, for example, require administration of varying amounts of insulin throughout the day to control their blood glucose levels. In recent years, ambulatory insulin infusion pumps have emerged as a superior alternative to multiple daily syringe injections of insulin. These pumps, which deliver insulin at a continuous basal rate as well as in bolus volumes, were developed to liberate patients from repeated self-administered injections, and allow them to maintain almost normal routines. Delivered volumes must be precise and in accordance with a programmed delivery schedule because an overdose or under-dose of insulin could be fatal.
Several conventional ambulatory insulin infusion devices are currently available on the market. One configuration of these devices relates to a miniature skin adherable infusion device, also referred to as a “dispensing patch”. It is lightweight, small in size (discreet), and has no tubing. Some dispensing patches use peristaltic metering mechanisms. An example of such a dispensing patch is disclosed in the co-owned, co-pending U.S. patent application Ser. No. 11/397,115 and International Patent Application No. PCT/IL06/001276, the disclosures of which are incorporated herein by reference in their entireties. Other dispensing patches may employ syringe pumps, examples of which are disclosed in co-owned, co-pending International Patent Application No. PCT/IL2008/000641, filed May 11, 2008 and entitled “A Positive Displacement Pump” and U.S. Provisional Patent Application No. 61/123,509, filed Apr. 9, 2008 and entitled “Systems, Devices and Methods for Fluid Delivery”, the disclosures of which are incorporated herein by reference in their entireties.
A peristaltic mechanism typically includes a rotary wheel with rollers and a flexible delivery tube. The rotary wheel with rollers periodically squeezes the flexible tube and delivers the fluid in the direction of rotation of the rotary wheel. A stator provides a counter force against the rotary wheel and has a groove designed to hold the tube in place. The spring-loaded stator can change its position in relation to the rotary wheel. A revolution counter alerts the patient in cases of electro-mechanical dissociation, as disclosed in the co-owned, co-pending International Patent Application No. PCT/IL08/000,642, filed May 11, 2008, and entitled “Methods and Apparatus for Monitoring Rotation of an Infusion Pump Driving Mechanism,” the disclosure of which is incorporated herein by reference in its entirety.
The use of a peristaltic mechanism maintains fluid sterility because the rotary wheel only touches the outer surface of the delivery tube, avoids pressure fluctuations because the delivery tube is continuously squeezed, and eliminates the need for a check valve.
Typically, an accurate and constant flow rate is to be delivered into the body of a user. However, some pumping mechanisms, such as a peristaltic pumping mechanism, have a variable flow rate, thus limiting the pumping mechanism as applied to ambulatory insulin infusion pumps. The peristaltic mechanism delivers the fluid in a series of pulses or surges, also referred to as a pulsation. During a rotary wheel cycle, the flow rate changes according to the relative position of the rollers and the stator. Moreover, no flow or backflow occurs when each roller disengages the stator. These pulsations are of no consequence to most applications, but their influence is significant when a low flow rate is needed, such as for example, during basal insulin delivery by a portable pump. The pulsation frequency is equal to the frequency of passing of successive rollers in contact with the delivery tube causing to a variable ratio between the amount of fluid delivered and the relative position of the pumping mechanism. This may cause inaccuracies and variation, especially when the pumping mechanism is activated without completing a full period, such as for example during low basal delivery. And when the dispensing device is operated according to the pumping mechanism periods, the ability to control and program the fluid delivery schedule is reduced and the effectiveness of the therapeutic treatment may be hampered. Referring to a peristaltic pumping mechanism as an example, the volume delivered by the change of the relative position of the rotary wheel (also referred to as a “flow rate”) may be affected by various parameters, including the delivery tube's mechanical characteristics (e.g., inner and outer diameters and polymer characterization), the rotary wheel diameter, the number of rollers, the stator's diameter, and the stator's spring. The flow rate can also be influenced by other moving parts of the peristaltic pumping mechanism, including without limitation, gears, the shaft, the motor, the steady stator, and the spring-loaded stator.
Conventional systems use mechanical means to reduce pulsations. An example of such mechanical means is discussed in U.S. Pat. No. 6,099,272 to Armstrong et al., which discloses a torque control cam that increases the minimal torque provided by the pump. In addition, U.S. Pat. No. 4,568,255 to Lavender et al. discloses elongated sloped-sweep vanes that increase pressure on the tube. Both mechanisms require high energy, a large motor and a powerful battery.
The reliability of an infusion pump can be enhanced by monitoring the flow rate of the therapeutic fluid. Conventional flow meters employed in infusion pumps are heavy and bulky and cannot precisely monitor low flow volumes. In other words, they do not allow accurate monitoring of the flow rate. An example of such a measurement mechanism is disclosed in International Patent Application No. PCT/US2002/038822 to Sage et al., wherein flow is monitored by optically detecting changes of the fluid refraction index caused by artificially-induced heat. This method is inaccurate for monitoring low flow rates and therefore cannot prevent potential deterioration of the therapeutic fluid or other fluid delivered to the body.